1. Field of the Invention
The present invention relates to a valve timing control apparatus for controlling valve timing of a valve that is opened and closed by a camshaft through torque transmitted from a crankshaft in an internal combustion engine.
2. Description of Related Art
Conventionally, a hydraulic valve timing control apparatus, having a housing serving as a first rotor rotatable with a crankshaft and a vane rotor serving as a second rotor rotatable with a camshaft, is widely used. As one type of such valve timing control apparatus, JP-2006-63835A discloses an apparatus for valve timing control, which supplies working fluid to advance chambers or retard chambers formed in a rotation direction between shoes (lobes) of the housing and vanes of the vane rotor in order to drive the camshaft in an advance direction or in a retard direction with respect to the crankshaft.
More particularly, in the apparatus disclosed in JP-2006-63835A, a spool valve controls a connection state of each of a supply passage and a drain passage with a corresponding one of the advance chamber and the retard chamber. In the above, the supply passage allows working fluid supplied from a pump to flow therethrough, and the drain passage allows discharged working fluid to flow therethrough. For example, when a phase of the camshaft with respect to the crankshaft (hereinbelow, referred to as an “engine phase”) is changed in the advance direction, the supply passage is connected to the advance chamber, and the drain passage is connected to the retard chamber by movement of a spool in the spool valve. On the other hand, when the engine phase is changed in the retard direction, the connection relations of the respective passages are reversed by the movement of the spool in the spool valve.
As disclosed in JP-2006-63835A, generally, in a valve timing control apparatus, variable torque moves or displaces the camshaft in the advance direction and the retard direction with respect to the crankshaft. The variable torque is always caused by a reaction force or the like of a valve spring for a valve that is opened and closed with the camshaft during running of the internal combustion engine. The torque varies in correspondence with rotational state of the internal combustion engine.
Accordingly, for example, it is supposed an operational state, where the engine phase is changed in the advance direction and at the same time the variable torque is applied in a direction to advance the camshaft. In the above, the volume of the advance chamber is expanded or increased by the action of the above torque in the advance direction. As a result, when the amount of fluid supply from the pump is relatively small, the working fluid may become insufficient in the advance chamber disadvantageously. Accordingly, when the application direction of the variable torque is reversed, the shortage of the working fluid in the chamber may cause and the retard of the camshaft, and as a result, the response in the advance operation may be degraded disadvantageously.